The present invention relates to a heating device which utilizes induction heating, and more specifically, to a fixing device for fixing a toner image, which is used in an electronic photographic type color copier, color printer or the like, which uses toner as an image visualizing agent.
In a color copier or a color printer, of the electronic photographic type, a fixing device for fixing an output image, that is, a toner image, onto a sheet, is used, and the device employs the following popular method, that is, a metal cylinder within which a heat source such as a halogen lamp is disposed, is heated by the heat source with radiated heat, and the heat, maintained in the metal cylinder, is propagated to a sheet and a toner image (toner), so as to fix the toner (toner image) onto the sheet.
In the fixing device of the above-described type, an elastic roller member is pressed at a predetermined pressure onto a metal roller containing a halogen lamp within itself, and a sheet onto which a toner image is statically adhered is made to pass between these rollers, so as to heat the toner and sheet and at the same time, apply a predetermined pressure thereto. In this manner, part of the toner melted by the heat is fixedly attached to the sheet, and at the same time, toner particles which are not in contact with the sheet are bonded together, thus fixing the toner (toner image) onto the sheet.
However, in the fixing device which utilizes a halogen lamp, such as described above, the light and infrared radiation are radiated onto the metal roller from all circumferential directions as well as the longitudinal direction (that is, the axial direction), and thus the entire roller is heated. Therefore, in consideration of the loss of energy resulting when light is converted into heat, and the efficiency for propagating heat to the roller by heating the air within the roller, etc., the heat exchange efficiency is only 60 to 70%. Thus, the heat efficiency is at such a low level, and therefore a lot of energy is required. Further, it may take even several minutes in order to increase the temperature of the outer circumferential surface of the metal roller to a predetermined level.
Under these circumstances, recently there has been proposed a fixing device, in which a heating member and a film with a cylindrical form and having a heat resisting property, which can be conveyed as it is tightly attached to the heating member, are used. In this device, a material to be heated is tightly attached to a film, and it is conveyed together with the film, so as to impart the heat energy of the heating member, from the film to the material to be heated.
With regard to the fixing device using such a film, as described above, it is essential to maintain the temperature of the heating member uniformly in the longitudinal direction, and therefore the uniformity of the thermal characteristics of the fixing device is required in the manufacture process and high-efficiency temperature control is required in the operation. Therefore, the production cost for the device is inevitably increased.
In addition, in high-speed copiers which process a great number of copies per unit time, the heating time is shorter than usual types, and therefore the heating member must be of a high quantity type (large capacity), which requires a much higher consumption power. Further, as the total power consumption becomes excessively large, which easily causes a problem in terms of safety standards.
In order to solve such problems entailed in the fixing operation using the heater, or using of a film, so far, there have been proposed fixing devices which utilize an induction heating method, as discussed in Jpn. Pat. Appln. KOKAI Publication No. H09-258586, H09-106207, H08-77620 and the like.
Jpn. Pat. Appln. KOKAI Publication No. H09-258586 discloses a fixing device operating on such a method that an electrical current is made to flow through the induction coil prepared by winding a coil wire around a core provided along the rotation shaft of the metal roller, and thus an induction current is generated in the roller, thus making the metal roller to generate heat from itself.
Jpn. Pat. Appln. KOKAI Publication No. H09-106207 discloses a fixing device of an induction heating type, a plurality of induction coils are set in a longitudinal direction within a metal roller made of a thin material, such that two coils located in a central portion are connected in parallel, and coils at both ends are connected in series with respect to the coils at the center, thus increasing the amount of heat generation at both ends of the metal roller.
Jpn. Pat. Appln. KOKAI Publication No. H08-76620 discloses a fixing device consisting of an electro-conductive film in which magnetic field generating means is contained, and a pressure roller brought in tight contact with the electro-conductive film, in which the electro-conductive film is made to generate heat, and thereby toner on a recording medium is fixed thereon as the medium is conveyed between the conductive film and the pressure roller.
It should be noted here that in the fixing device of the induction heating type, the temperature of the end portions of the heat (metal) roller is known to become lower than that of the central portion. Further, the length of the heat roller is specifically determined in accordance with the width of a sheet having the maximum size copyable. As a result, when copying is repeated continuously with sheets of a smaller width, the temperature of the central portion of the temperature distribution in the longitudinal direction of the roller becomes lower.
As discussed in its specification, the technique of Jpn. Pat. Appln. KOKAI Publication No. H09-106207 makes it possible to suppress the temperature of the end portion of the metal roller decreasing to a level lower than the temperature of the central portion.
However, in the case where the entire region of the roller in the longitudinal direction is maintained all times at a constant temperature, it is inevitable that heat is applied to unnecessary excessive portions on both ends of the roller while copying is continued on sheets of a small width, which is nothing but a waste of energy.